Radiation • Group 3: Manabe and Wetherald (1975) and Trenberth and Fasullo (2009) – What is the energy balance of the climate system? How is it altered by greenhouse gases? How does it reach a new equilibrium? What does that new equilibrium look like? (Jenna and Josh) – Why does the stratosphere cool? (Peng)
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Radiation Group 3: Manabe and Wetherald (1975) and Trenberth and Fasullo (2009) – What is the energy balance of the climate system? How is it altered by.
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Radiation
• Group 3: Manabe and Wetherald (1975) and
Trenberth and Fasullo (2009)
– What is the energy balance of the climate system? How is it altered by greenhouse
gases? How does it reach a new equilibrium? What does that new equilibrium look
like? (Jenna and Josh)
– Why does the stratosphere cool? (Peng)
– How does the T&F differ from the simple radiative‐
convective adjustment we discussed in class? (Ryan)
– How do we know and how well do we know the energy budget? That is, can we test these ideas
against observations? (One member of the group may have to do some additional research on this)
(Thania and Shaun)
What is the energy balance of the climate system?
RT: Net radiation at top of atmosphere
ASR: Absorbed Solar Radiation – affected by planetary albedo and cloud cover
OLR: Outgoing longwave radiation – affected by surface temperature and cloud cover
RT = ASR - OLR
How is the energy balance altered by greenhouse gases?
Greenhouse gases increase radiative forcing → Higher surface temperatures, More evaporation → More water vapor in atmosphere
With higher surface temperatures comes less snow and ice, reducing the planetary albedo
The role of clouds is examined in the paper by Trenberth and Fasullo (2009)
Trenberth, K. E., and J. T. Fasullo (2009), Global warming due to increasing absorbed solar radiation, Geophys. Res. Lett., 36, L07706, doi:10.1029/2009GL037527.
Changes in net radiation (top), -OLR (middle), and ASR (bottom) from 1960 to 2100 using 13 CMIP3 models. On the right are averages over the period of 1950 to 2100.
Reaching a New Equilibrium
1. Higher Surface Temps
2. Increased es
1. Increased evaporation
2. Increased water vapor
3. Positive greenhouse feedback
3. Increase in OLR due to differential tropospheric warming
1. Partially offsets water vapor feedback
4. Reduction in albedo
New Equilibrium
OLR and ASR need to balance for Equilibrium
OLR increases due to increasing temperature
ASR increases due to reduced albedo
From 1950-2000
Decrease in OLR, net warming
By 2050, in T&F
Increase in OLR due to warming balances greenhouse effect
Why Does the Stratosphere Cool?
http://www.atmosphere.mpg.de/enid/20c.html
H20CO2
σT14
σT24
σT04
To
Ts
For the Visual Learners:
Atmospheric Window
Troposphere
Stratosphere
CO2
Space
1) More greenhouse gas is added to the tropospheric layer8) To balance this increased output, the stratospheric temperature must decrease 7) From the perspective of space, the radiation output from the stratosphere increases6) The stratosphere therefore absorbs and re-emits more radiation5) From the perspective of the stratosphere, more radiation is emitted from the top of the troposphere4) Tropospheric temperature must increase in order to balance this3) From the perspective of space, outgoing radiation is reduced (due to smaller atmospheric window)2) Outgoing radiation is absorbed more efficiently (the atmospheric window becomes smaller)
The differences between models used in book and in paper
Book (Wallace & Hobbs, 2006) Paper (Manabe & Wetherald, 1967)
How do we How do we knowknow the energy budget? the energy budget?
The global annual mean Earth’s energy budget for the Mar 2000 to May 2004 period (W/m²). (Trenberth, et al. 2009)
““know”know”qualitatively: physical processes involved in the transformation and transfer of the radiation and heat fluxesquantitatively: values of the radiation and heat fluxes
TOA (SW, LW) 31. Land based observations: observational
data (vertical distributions of T, humidity (rh and q), P compute the terms by equations (Manabe et al., 1964; Manabe et al., 1967)
1. Top of the Atmosphere Fluxes1. Satellite Observations – Balanced within ~3W/m2 (Kiehl et al. 1994)
1. Solar Irradiance varies from satellite to satellite – 1365 to 1373 W/m (Ardanuy et al. 1992)
1.- Earth Radiation Budget Experiment (ERBE)* 1985-1989 - Designed to measure global albedo, fluxes, and solar radiation
2. Solar Irradiance varies throughout time2.Associated with the buildup of greenhouse gases 3.Associated with heat changes in heat storage in climate systems (i.e., El Niño events)
NASA’s Modern Era Retrospective-analysis for Research and Applications (MERRA)
Global Energy Budget (cont.)
How Well Do We Know? (cont.)
1. Longwave Radiation Fluxes – Values differ by 21 W/m2 (Kiehl & Trenberth, 1997)1. Model Calculations 2. Depends on temperature and efficiency of gaseous absorber at a given wavelength (Kiehl & Ramanathan, 1983)
2. Shortwave Radiation Flux –1. Model Calculations –
2. Albedo uncertainties are an issue. 1. How significant a roll does water vapor presence play? 1. To what extent do cloud and aerosol overlaps affect the balance?